Coriolis flow measuring devices are, in such case, insertable as In-line-measuring devices into a pipeline, such as, for example, a process line of an industrial plant. Determinable via Coriolis flow measuring devices is at least one parameter, such as, for example, a mass flow, a density, a viscosity, etc., of the fluid flowing in the pipeline. For this, the measuring transducer includes at least one measuring tube, which, during use, is excited to oscillations by an exciter mechanism. In such case, it is especially known to use curved measuring tubes.
As is known, curved measuring tubes can, in the case of being excited to bending oscillations according to a first particular oscillation form (wanted mode), effect Coriolis forces in the fluid flowing through. As the first particular oscillation form of the curved measuring tube, in which the measuring tube is excited, the fundamental mode of the bending oscillation is usually selected. In the fundamental mode of the bending oscillation, the measuring tube moves in a pendulum-like manner at a lowest possible resonance frequency about an imaginary longitudinal axis of the measuring transducer in the manner of cantilever clamped at one end. Due to the Coriolis forces occuring, oscillations are superimposed with an equal frequency on the wanted mode according to at least one second particular oscillation form (Coriolis mode). In the Coriolis mode, the measuring tube also performs rotary oscillations about a vertical axis arranged perpendicular to the longitudinal axis, the vertical axis especially extending in the plane defined by the curved measuring tube. Due to the superpositioning of the wanted and Coriolis modes, the oscillations of the measuring tube, registered on the inlet side and outlet side by means of a sensor arrangement, have a measurable phase difference. This phase difference is, among other things, dependent on the mass flow.
An important aspect during use of Coriolis flow measuring devices is the fact that these are largely decoupled from the respective connected pipeline, so that, if possible, no oscillations are conveyed into the pipeline, as oscillations introduced into the pipeline lead, among other things, to reflections, which then, in turn, can negatively influence the measurement signal. Accordingly, the requirement is placed upon a measuring transducer that it have an even balance. This means that, if possible, no forces and/or oscillations of the measuring transducer are transferred into the adjoining pipeline. For this, various concepts are already known.
One known concept is to provide two parallelly flowed through, curved measuring tubes, which, as a rule, are arranged symmetrically to one another with respect to a plane extending between the two measuring tubes. In the case of this concept, on the inlet side and outlet side of the measuring tubes, distributor pieces are in each case required, which are comparatively complex to manufacture and which, depending on the fluid used, can display a tendency toward accretion formation and toward clogging.
Along with that, it is known to provide, in addition to a single, flowed-through, curved measuring tube, a counteroscillator, which likewise is excited to oscillations. The counteroscillator is, in such case, to be matched to the oscillation characteristics of the measuring tube in such a manner, that as even a balance of the measuring transducer as possible is produced. The counteroscillator is, in such case, matched as a rule to a reference condition, which is formed by a measuring tube filled with water. If the measuring tube is flowed through by a fluid or medium of a different density, as the density difference increases, the balance becomes more uneven, and the accuracy of the measurement is reduced. From this arises another requirement for the measuring transducer with its counteroscillator, namely that these have a high accuracy of measurement and an even balance across as broad a density range for the particular fluid as possible.
In WO 2007/074014 A1, a concept is described, in the case of which the counteroscillator is formed by two counteroscillator plates arranged laterally to a curved measuring tube. In WO 2002/099363 A1 a concept is described, in the case of which is provided lateral to a curved measuring tube a counteroscillator extending essentially parallel to the measuring tube. Easily enabled both in the case of the measuring transducer from WO 2007/074014 A1 as well as in the case of the measuring transducer from WO 2002/099363 A1 is a pendulum-like movement of an entire inner part—which at least the measuring tube and the counteroscillator have—about a longitudinal axis of the measuring transducer under a torsion of two connecting tube pieces adjoining on the measuring tube.
In the publication DE 10 2007 051 420 A1, a Coriolis mass flow measuring device with a straight measuring tube made of a corrosion-resistant material is described. Stabilizing elements, which are coupled with the measuring tube via add-on parts connected directly with the measuring tube, are, in such case, formed from a different metal than the measuring tube. The metal of the stabilizing elements has a coefficient of thermal expansion matched to the metal of the measuring tube.
If the fluid to be measured is formed by a corrosive medium, the components of the measuring transducer which come in contact with the corrosive fluid must then be formed from corrosion-resistant material. Especially in the case of highly corrosive media, tantalum is very well-suited as such a corrosion-resistant material. Tantalum is, however, comparatively expensive. In the case of use of a counteroscillator, this counteroscillator must display as close to the same oscillatory behavior as the curved measuring tube as possible in the case of the different use conditions. This can, among other things, be achieved in that the counteroscillator is formed from the same material as the measuring tube and has a similar or identical geometry. If an expensive material and/or a material difficult to process during manufacturing is generally used as the material for the measuring tube, a corresponding embodiment of the counteroscillator can lead to high material costs and/or to a relatively high manufacturing effort, which then leads to high costs.